Prepend certain lines of file

Question

I\'d like to create a script to comment out lines of my Mac OS X hosts file that contain .com. And also one to reverse it.

So this:

127.0.0.         


        

Answer 1

sed '/\.com/s/^/#/' < hosts

Interpretation:

• /\.com/ - only perform the rest of the command on lines matching this regex
• s/^/#/ - insert # at the beginning of the line

If you want to replace the original file, use sed's -i option:

sed -i.bak '/\.com/s/^/#/' hosts

This will rename hosts to hosts.bak and create a new hosts with the updated contents.

To undo it, use:

sed -i.bak '/^#.*\.com/s/^#//' hosts

Answer 2

With awk

awk '$2 ~ /.com/{$0 = "#"$0;}{print}' temp.txt

Output

#127.0.0.1    foo.com
#127.0.0.1    bar.com
127.0.0.1    baz
127.0.0.1    qux